1. Field of the Invention
This invention relates to utilization of solar energy and more particularly, to coating compositions for selective solar absorption which can provide effective selective solar absorbing coatings for solar collectors and also to articles obtained from these coating compositions.
2. Description of the Prior Art
A variety of black absorber layers such as obtained from ordinary black paints which have been utilized in the field of domestic hot water supply have been found to efficiently absorb incident solar radiation but to readily emit once absorbed thermal energy.
As is well known in the art, solar collector surfaces or layers should preferably have such characteristics as not only to absorb solar energy in the ultraviolet, visibile and near-infrared regions as much as possible, but also to minimize the energy emitting from the surface whose temperature has been increased by the absorption of the solar energy.
Owing to the substantial absence of materials having these solar selective absorption properties, attempts have been made to achieve a high degree of solar selectivity by the use of various combination systems.
Typical of prior-art techniques is formation of absorbers on many metal substrates which are a good reflector in the infrared region. The absorber should well absorb solar radiation at the short wavelengths of, say, 0.3-2.0 micrometers without impairing the reflectivity of metal substrate. For this purpose, there are used black copper, black chrome, black nickel and the like. In practice, the technique has mainly been applied to systems of higher temperatures than 92.degree. C. at which the solar absorptance of the black body will equilibrate with the thermal emittance loss. In this system, the ratio of the solar absorptance, .alpha., to the thermal emittance, .epsilon., is one of important factors. Lower thermal emittances result in lower solar absorptances and vice versa. In order to meet the requirement that the ratio, .alpha./.epsilon., should be higher, it is the general practice that the selective absorption films of black copper, black chrome and the like are controlled at levels of a solar absorptance of 0.90 and a thermal emittance of 0.15.
For domestic hot water supplies, the absolute value of .alpha. is more important as the solar selective absorptivity characteristic than the ratio, .alpha./.epsilon., since the surface temperature of domestic collector systems is at most as low as 75.degree. C. and thus the radiant energy level from the surface becomes low. In this connection, however, in flat plate collector systems for hot water service, the thermal radiative loss from the collector surface may reach even 20-30% of the total loss of the system, so that it will be effective in improving the collector efficiency to suppress the thermal emittance from the collector surface to a low level.
The first demand for apparatuses utilizing solar energy is economy because they are used for energy saving. The selective absorption films of, for example, black chrome and the like mentioned hereinbefore involve a difficulty when applied for domestic hot water supply since their plating equipment is usually large in scale and thus the processing cost is increased.
Some attempts have been made to produce selective absorption paint films or coatings which have advantages such as ease of application, low production cost and availability of large coating areas, among which there are well known studies of paints which comprise semiconductor pigments such as Ge, Si, PbS or the like dispersed in silicone resins or the like. For instance, these studies have been reported in "High absorptivity solar absorbing coatings" by D. M. Mattox and R. R. Sowell, J. Cac, Sci. Technol., Vol. 11, No. 4, pp 793-796 (1974) and "Solar absorptance and emittance properties of several solar coatings" by R. B. Pettit and R. R. Sowell, J. Vac. Sci. Technol. Vol. 13, No. 2, pp 596-601 (1976).
In these paint coatings, however, the absorptance, .alpha., is over 0.9 but the thermal emittance, .beta., is in the range of 0.70-0.90 in all the samples, thus the selective absorptivity is poor. Pettit et al stated in the above-mentioned literature that the high emittance is due to the fact that when the film of the silicone binder itself exceeds 10 micrometers in thickness, the emittance increases. In order to overcome this disadvantage, there has been proposed an improved method in which the ratio of semiconductive particles to silicone resin increases so as to decrease the content of the silicone resin in paint film. However, this method is also disadvantageous in that (1) the viscosity of paint increases, (2) mechanical strengths of the paint film are lowered, and (3) adhesion between the paint and metal substrate is lowered.
As particularly shown in these researches, selective absorption films obtained by prior-art paint coating techniques invariably present the following technical problems: (a) if the thickness of paint film is reduced, the emittance is lowered but undesirably the solar absorptance is also lowered; (b) as for the film thickness, even though it would have experimentally been confirmed that a film thickness of 1 micrometer could give relatively satisfactory results with respect to selective absorptivity, coating techniques are not known for stably forming a uniform 1 micrometer thick film; and (c) such a thin film of about 1 micron, if obtained, would exhibit only poor and unreliable physical properties.